Sounds like you're ready to learn.
I see 4 corner idle. With that cam, the engine is probably making 1/2 decent vacuum at idle. So plan on using very little idle mix. In fact if you shut the secondary idle mix screws all the way, it might might be a good starting piont. Then crack them as needed after adjusting the primaries and fiddle with all four to get the best result. Best result means putting it in gear and getting the least amount of rpm and vacuum drop.
As far as secondary transfer slot goes. Yes there should be a small screw to adjust the secondary idle position. Holley put them so you could only adjust them with the carb off (like in photo). Barry Grant - I don't know. But you are correct, secondary transfer slots should not show. or at least hardly show.
Re: .020 on the primary. Barry Grant carbs have a rep for wider and longer t-slots than old school holleys. Regardless don't feel locked into the number. Some adjustment is OK. When its too far off, you'll notice a delay or a bog when gently pulling away from a stop sign or such..
There's probably more, but that's a good starting point.
Some barry Grants have 'idle eze' air bypass adjustment. The purpose is to allow the throttles to start in the .020ish position without resorting to drilling holes in the blades or cranking open the secondary blades. I think Bill Jones came up with the idea.